Heat-shrinkable pressure-sensitive adhesive tape

ABSTRACT

Heat-shrinkable electrically insulative pressure-sensitive adhesive tape made from 1) a backing selected from polyethylene terephthalate and polyvinyl chloride backings that shrink at a useful elevated temperature at least twice as much in length as in width and shrink within the ranges of 10-50 percent in length and 0-15 percent in width and 2) a pressure-sensitive adhesive that exhibits a high shear strength and a high viscosity.

United States atent n91 Tomita HEAT-SHRINKABLE PRESSURE- SENSITIVEADHESIVE TAPE [75] Inventor: Jun Tomita, St. Paul, Minn.

[73] Assignee: Minnesota Mining and Manufacturing Company, St. Paul,Minn.

[22] Filed: Feb. 12, 1971 [21] Appl. No.1 115,093

Related U.S. Application Data [63] Continuation-impart of Ser. No.513,537, Dec. 13,

1965, abandoned.

[52] U.S. Cl ..117/7, 1l7/68.5, 117/76 A, 117/122 PF, l17/138.8 F,ll7/l38.8 UA

[51] Int. Cl ..C09j 7/02, 1305c 3/107 [58] Field of Search ..117/122 PF,138.8 F, 76 A, 117/68.5, 138.8 UA

3,236,677 2/ l 966 Bradstreet ..1l7/68.5 3,405,833 10/1968 Salzinger etal. ..215/96 2,790,286 4/1957 Snyder ..53/41 2,553,816 5/1951 Ebel....117/122 3,241,662 3/1966 Robinson et al... ..206/59 3,197,326 7/1965Webber ..117/76 2,973,286 2/1961 Ulrich ....1 17/122 3,251,713 5/1966Crone ..117 122 Primary Examiner-William D. Martin AssistantExaminerBernard D. Pianalto Attorney-Kenney, Alexander, Sell, Steldt &DeLal'lunt [57] ABSTRACT Heat-shrinkable electrically insulativepressure-sensi tive adhesive tape made from 1) a backing selected frompolyethylene terephthalate and polyvinyl chloride backings that shrinkat a useful elevated temperature at least twice as much in length as inwidth and shrink within the ranges of 10-50 percent in length and 0-15percent in width and 2) a pressuresensitive adhesive that exhibits ahigh shear strength and a high viscosity.

8 Claims, No Drawings HEAT-SHRINKABLE PRESSURE-SENSITIVE ADHESIVE TAPEThis application is a continuation-in-part of application Ser. No.513,537, filed Dec. 13, l965and now abandoned.

The heat-shrinkable tape of the present invention, which is the firstknown pressure-sensitive adhesive tape having useful properties ofheat-shrinkability, provides superior insulative coverings when wrappedaround an article and heat-shrunk. Preferred kinds of the new tape,which are both widthand lengthshrinkable, are wrapped around cylindricalelectrical components in a width greater than the axial dimension of thecomponent and then shrunk down to cover at least a portion of thetransverse end of the component. Another advantage of a heat-shrinkabletape of the invention is that, in general, such a tape provides atighter, more firm covering than that provided by earlier non-shrinkabletapes.

Tape of the invention is also useful to provide physi' cal protectionand perform holding functions. The tape may be used, for example, totightly hold together a bundle of electrical conductors or to provide atight seal over the edges of a container closure.

A useful heat-shrinkable pressure sensitive adhesive tape, it has beenfound, must have a controlled shrinkage, shrinking substantially more inthe longitudinal direction than in the width direction. It is preferablethat the tape show some shrinkage in the width direction to accomplishthe best coverage of the transverse end surface of the article wrapped.The tape should shrink its prescribed amount at a predeterminedtemperature, and after shrinking that amount should be dimensionallystable at the shrinking temperature or lower. Further, the backing ofthe tape should be flexible and tough, moistureand solvent-resistant(i.e., resistant to typical organic solvents), and should have biaxialstrength at the shrinking temperature os that the tape withstands thetensions of the shrinking operation, especially if it is drawn down overprotrusions. The heat-shrinkable backing may include materials such asnon-woven webs that do not interfere with the shrinking process.

It has been found that the necessary features are obtained with a tapethat includes a strong, tough backing that shrinks upon exposure to auseful elevated temperature (such as 200-400 F.) at least twice as muchin length as in width and within the range between about 10 and 50percent in length and between about and 15 percent in width, preferablybetween about 15 and 30 percent in length and between about 5 andpercent in width. Preferably, this backing is a biaxiallyoriented,biaxially-differentially heat-shrinkable polyethylene terephthalate filmas disclosed in a patent of Merle L. Erickson, U.S. Pat. No. 3,631,899.

Since the tape is first wrapped in contact with a surface in theconventional manner and then shrunk, special requirements exist for theadhesive. The adhesive must be firmly united to the tape backing andsufficiently cohesive to avoid excessive shearing when shrinkage forcesdevelop in the tape upon its exposure to heat. If the adhesive is tooweak the tape may unwind and if the adhesive shears excessively the tapeshrinks and leaves an exposed sticky dust-collecting layer of adhesive.Further, if the adhesive is too soft, the end of the shrunken tape willlift away from its surface of application.

The necessary internal strength and firmness standards of the adhesivematerial can be established by shear and viscosity tests. Oneappropriate shear test is the method described in ASTM Dl000-64, section57(a) entitled Bond Separation During Cure, with the only deviationbeing the use of a IOO-gram weight in place of a SO-gram weight.l-lalf-inch-wide strips of tape based on an inert non-stretchy,non-shrinkable film such as biaxially-oriented and highly heat-setpolyethylene terephthalate film are fastened together to form a k inchby k inch adhesive-to-adhesive lap joint. A IOO-gram weight is fastenedto one end of one strip and the specimen is hung by the other end of theother strip in an oven maintained at C with the weight freely suspended.The time in minutes for the bond to separate by shearing at thistemperature is the shear strength. Using this test as an indication,adhesives that exhibit a shear strength value of 60 minutes or more,preferably 500 minutes or more, will have sufficient internal strengthand be firm enough to function on a heat-shrinkable tape backing.

An appropriate viscosity test is conveniently run with the adhesivecoated on an inert, non-sticky backing such as biaxially-oriented andhighly heat-set polyethylene terephthalate film. The adhesive thicknessis in the range of 1.0 to 1.5 mils and the film backing is conveniently2.0 mils. A V4-inch-Wide strip of tape is wrapped 360 around a freelyrotatable stainless steel rod of 0.238 centimeter radius with one end ofthe tape extending to a stationary clamp. A constant torque is appliedto rotate the rod to apply a shear stress over the full wrap, and therotational velocity is measured. The test is repeated at severalconstant torques, and a plot is made of torque vs. velocity from whichthe angular velocity is taken at a shear rate of 1 $66.". The viscosityV in poises at the shear rate is calculated using the equation where Tis the torque in gram-centimeters, t is the thickness of the adhesive incentimeters, and w is the velocity in degrees per minute. Using thistest as an indication for sufficient internal strength, adhesives shouldhave a viscosity of at least 1.0 X 10 poises to be firm enough for useon shrinkable film backings.

Some of the pressure-sensitive adhesives that have been found to performsatisfactorily in heat-shrinkable tapes of the invention include theclass of acrylatebased pressure-sensitive adhesives. In general, thisclass of adhesive includes polymers based on acrylate monomersconsisting of monomeric acrylic acid esters of non-tertiary alkylalcohols, the molecules of which have from one to 14 carbon atoms,usually between four and 12 carbon atoms. A mixture of differentacrylate monomers may be included, but at least a major proportion ofthe alcohol molecules of the acrylic acid ester should havecarbon-to-carbon chains of at least four carbon atoms terminating at thehydroxyl oxygen atom. The acrylate monomers may be copolymerized with aminor proportion of ethylenically unsaturated monomers having strongpolar groups such as acrylic acid. These adhesives have been found tohave the necessary firmness and internal strength at high temperature.Another useful class of adhesives are adhesives of the rubber-resin typewhich include natural or synthetic elastomers and a tackifying resin andare made thermosetting such as by inclusion of heat-reactivephenoleformaldehyde resin.

As previously noted, biaxially-oriented polyethylene terephthalate filmis a preferred backing material. This film is known to have excellentelectrically insulative properties. In addition, polyethyleneterephthalate maintains its strength in a condition of thermal exposureand resists melting, properties which are useful in themselves andespecially useful in heat-shrinkable tape to resist rupture during ashrinkage process. For example, tape of the invention is often caused toquickly reach shrinking temperature by blowing air over the tape that isat a temperature higher than the shrinking temperature of the tape.Thus, while the best shrinking temperature for polyethyleneterephthalate generally is about 300 F., areas of the film maytemporarily reach a higher temperature. But the high softening point ofpolyethylene terephthalate assures that the film will remain strong andnot rupture during the shrinking operation.

Biaxially-oriented, biaxially-differentially heatshrinkable polyethyleneterephthalate may be manufactured according to the processes taught inthe previously noted patent of Merle L. Erickson. This film can beprepared to have the controlled shrinkage set forth above as desirablefor heat-shrinkable tape. The amount of shrinkage desirable dependssomewhat on the size of the object being wrapped, larger objectsgenerally requiring a higher shrinkage.

Unplasticized polyvinyl chloride film may also be used as the backingfor useful heat-shrinkable tape. While the softening point of polyvinylchloride is lower than polyethylene terephthalates, it is moderatelyhigh, and polyvinyl chloride has other good properties for anelectrically insulating tape. For best results the backings of tapes ofthe invention may be primed so as to firmly anchor thepressure-sensitive adhesive on the backing. In addition, the backingscan be given a lowadhesion backsizing as is well known.

The invention is further illustrated in the examples which follow.

EXAMPLE 1 Tape was prepared using one-mil-thick, biaxiallyoriented,biaxially-differentially heat-shrinkable polyethylene terephthalate filmprepared in the manner taught in the previously noted patent of Merle L.Erickson and having shrinkages at 150 C of 31 and 14 percent in lengthand width, respectively. The film was knife-coated on one side, in a1.1-mil dry thickness, with an adhesive that consisted of a copolymer of90 parts isooctyl acrylate and parts acrylic acid. The adhesive wasapplied as a -percent-solids solution in a mixture of ethyl acetate andheptane solvents, and after coating, was dried in an oven at atemperature of 66 C. The other side of the film was coated with a thinuniform coat of a low-adhesion backsizing of the polyvinyl N-alkylcarbamate type described in U. S. Pat. No. 2,532,011 and was applied asdescribed therein. This backsize, applied from a 5 percent solution intoluene, was also dried at 66 C. In the tests described above, theadhesive had an inherent viscosity of 1.6 and a shear strength greaterthan 1000 minutes. The adhesion of the tape was found to be 35ounces/inch of width and the tape exhibited shrinkages in length andwidth at 150 C of 32 and 14 percent respectively. The tape was wrappedaround objects such as coils and then shrunk by heating to provideexcellent tight wrappings around the objects. Some of the samples wereheated in an oven in which the temperature was between 150 and 175 C andothers were heated by a short exposure to a hot air stream from a hotair gun at a temperature of about 200 C. There was no unwinding orlifting of the tape ends during or after the heat exposure.

EXAMPLE 2 Biaxially-oriented, biaxially-differentially heatshrinkablepolyethylene terephthalate film like that in Example 1, but exhibitinglength and width shrinkages at 150 C. of 20 and 7 percent respectively,was used to prepare tape. The film was coated in a thickness of 0.8 milwith a pressure-sensitive adhesive based on a copolymer of 90 partsfusel oil acrylate (in which 55 percent of the acrylic acid esters areof primary amyl alcohols, 15 45 percent of primary butyl alcohols, and 05 percent of n-propyl alcohol) and 10 parts acrylic acid. This adhesivehad a shear strength value in the above test of greater than 1000minutes. The other side was coated with the same backsizing as inExample 1 and the film was then dried at 66 C. This tape exhibited anadhesion of 38 ounces/inch of width. It had shrinkages of 21 and 6.2 percent in length and width respectively at 150 C. The shrinkage availablein this tape makes it particularly well suited for wrapping andshrinking around can-enclosed capacitors having diameters between aboutV4 and lk inches.

EXAMPLE 3 Tape was made from retensilized film that had been prepared bystretching lengthwise between about 1.4 and 1.5 times at 82 C a1.5-mil-thick polyethylene terephthalate film that had been originallybiaxially oriented about three times and heat set at 204 C. Thisconverted the original 1.5-mil film, which had shrinkages of less thantwo percent in both length and width, to a one-mil film having lengthand width shrinkages of 25 and 1.3 percent respectively at 150 C. Thisfilm was coated on one side with a crude rubber primer solution, dried,and then exposed to ultraviolet radiation in the manner described in U.S. Pat. No. 3,188,266, and over this was coated in a 1.2-mil thickness afirm type thermosetting adhesive that had the following composition:

Parts by Weight Crude smoked rubber sheet 100 Polymerized terpene resin(Piccolyte S-1l5) 60 Wood rosin 20 Zinc resinate 5 Phenol-aldehyde resin(Bakelite CKR-l 634) 20 Zinc oxide t 20 Di-tert-amyl hydroquinone 2Heptane 500 This adhesive had a shear strength value of greater than1000 minutes and a viscosity of greater than X 10 poises. The coatedfilm was dried in an oven at a temperature of 66 C. The other side wascoated and dried with a low-adhesion backsizing like that of Example 1.The resulting tape product had length and width shrinkages of 22.5 and1.3 percent respectively and had an adhesion of 33 ounces/inch of width.When two layers of this tape were wrapped around a inchdiameter,l-inch-long, can-enclosed capacitor such that the tape extended 3/32inch beyond each end of the capacitor and shrunk by exposure to a streamof hot air from a hot-air gun at a temperature of 204 C., a tight neatlywrapped covering with the edges conforming to the transverse ends of thecapacitor, was obtained. There was no lifting of the ends of the tapeduring or after the shrinking operation.

What is claimed is:

1. Heat-shrinkable pressure-sensitive adhesive tape comprising (1) astrong tough flexible moistureand solvent-resistant backing selectedfrom biaxially oriented polyethylene terephthalate and polyvinylchloride backings that shrink when heated to a useful elevatedtemperature between about and 50 percent in length and between about 0and percent in width and at least twice as much in length as in width,and (2) firmly united to one side of the backing a pressure-sensitiveadhesive that is tacky cohesive and firm at room temperature and at saidelevated temperature and that exhibits a shear strength value in thedescribed test greater than 60 minutes and a viscosity in the describedtest greater than 1.0 X 10 poises.

2. A tape of claim 1 in which the backing shrinks between about 15 and30 percent in length and between about 5 and 10 percent in width and thepressure-sensitive adhesive exhibits a shear strength value in thedescribed test greater than 500 minutes.

3. A tape of claim 1 in which the film backing is biaxially-orientedbiaxially-differentially heat-shrinkable polyethylene terephthalate.

4. A tape of claim 1 in which the adhesive is a polymer based on anacrylate of a nontertiary alkyl alcohol.

5. A tape of claim 1 in which the adhesive is a firm thermosettingrubber-resin adhesive.

6. Heat-shrinkable pressure-sensitive adhesive tape comprising (1) astrong tough flexible moistureand solvent-resistant biaxially orientedpolyethylene terephthalate backing that shrinks when heated to a usefulelevated temperature between about 10 and 50 percent in length andbetween about 0 and 15 percent in width and at least twice as much inlength as in width, and (2) firmly united to one side of the backing apressure-sensitive adhesive that is tacky cohesive and firm at roomtemperature and at said elevated temperature and that exhibits a shearstrength value in the described test greater than 60 minutes and aviscosity in the described test greater than 1.0 X 10 poises.

7. A tape of claim 6 in which the film backing shrinks between about 15and 30 percent in length and between about 5 and 10 percent in width andthe pressure-sensitive adhesive exhibits a shear strength value in thedescribed test greater than 500 minutes at said.

elevated temperature.

8. A tape of claim 6 in which the adhesive is a polymer based on anacrylate of a nontertiary alkyl alcohol.

2. A tape of claim 1 in which the backing shrinks between about 15 and30 percent in length and between about 5 and 10 percent in width and thepressure-sensitive adhesive exhibits a shear strength value in thedescribed test greater than 500 minutes.
 3. A tape of claim 1 in whichthe film backing is biaxially-oriented biaxially-differentiallyheat-shrinkable polyethylene terephthalate.
 4. A tape of claim 1 inwhich the adhesive is a polymer based on an acrylate of a nontertiaryalkyl alcohol.
 5. A tape of claim 1 in which the adhesive is a firmthermosetting rubber-resin adhesive.
 6. Heat-shrinkablepressure-sensitive adhesive tape comprising (1) a strong tough flexiblemoisture- and solvent-resistant biaxially oriented polyethyleneterephthalate backing that shrinks when heated to a useful elevatedtemperature between about 10 and 50 percent in length and between about0 and 15 percent in width and at least twice as much in length as inwidth, and (2) firmly united to one side of the backing apressure-sensitive adhesive that is tacky cohesive and firm at roomtemperature and at said elevated temperature and that exhibits a shearstrength value in the described test greater than 60 minutes and aviscosity in the described test greater than 1.0 X 106 poises.
 7. A tapeof claim 6 in which the film backing shrinks between about 15 and 30percent in length and between about 5 and 10 percent in width and thepressure-sensitive adhesive exhibits a shear strength value in thedescribed test greater than 500 minutes at said elevated temperature. 8.A tape of claim 6 in which the adhesive is a polymer based on anacrylate of a nontertiary alkyl alcohol.